Understanding variation in complex trophic interactions is a major goal of community ecology and is necessary to predict the efficacy of biological control programs involving multiple natural enemies. Identifying factors that affect variation in the strength of trophic interactions, however, is often extremely challenging. We quantified variation in the strength of key trophic interactions in four agroecosystems (cotton, soybean, collards, and tomato) and used a comparative method technique to determine that positive species’ interactions (e.g., mutualisms) play a major role in determining the magnitude of important predator – prey interactions (e.g., ants and caterpillars) and the ultimate success of biological control in these crops.